Modern passenger vehicles frequently include a powertrain that includes an engine, a multi-speed automatically-shiftable or automatic transmission, and a differential or final drive. An automatic multi-speed transmission increases the overall operating range of the vehicle by permitting the engine to operate through its torque range multiple times without requiring an operator of the vehicle to manually select specific speed-ratios or gear ranges. The number of forward gear ranges or speed-ratios that are available in the transmission determines the number of times the engine torque range is repeated. An automatic transmission such as above may utilize an epicyclic or planetary gearing, or a parallel shaft mounted gear-train, such as in a Dual-Clutch transmission (DCT), for affecting specific transmission speed-ratios.
Modern automatic transmissions are typically controlled by an electronic controller, which may either be a dedicated transmission control unit (TCU) or powertrain control module (PCM) configured to control the entire powertrain, including the engine. The appropriate controller generally uses data provided by sensors from various vehicle systems to calculate how and when to change gears in the vehicle for optimum performance, fuel economy, and shift quality. Although such a controller is generally programmed to select the subject transmission's gear ratios automatically, frequently, modern automatic transmissions also offer to the vehicle operator the option of manual control over the selection of specific gear ratios.